In recent years fluidized beds have found many diverse uses in power generating systems and chemical processes. They have served as chemical reactors, particularly for finely divided materials; as incinerators for liquid, solid or gaseous substances; as pressurized or atmospheric, coal-, lignite-, petroleum-, peat-, wood- and/or paper-fired boiler or combustor units for power generation; and, as sites for various process treatments such as drying, baking, coating, oxidizing, etc.
Typically, fluidized beds which are in use today are static beds established when air or other fluidizing gas is introduced into a plenum chamber under pressure and forced upwardly through a diffusing medium (e.g., membrane, grate) to a superimposed chamber containing a particulate bed, of inert or reactive, finely divided, pulverulent solid material. Gas, forced upwardly through the diffusing medium into the fluidizing chamber under a sufficient predetermined pressure, fluidizes the particulates. The gas pressure required to accomplish this is determined, in part, by the nature and degree of fineness of the particulates to be fluidized. Other influencing factors are the depth of the bed and the size, number and design of the plenum chamber compartments and passages into the superimposed fluidizing chamber.
The rate at which a chemical reaction takes place in a fluidized bed between a solid material and a gaseous agent depends to a major extent on the rate at which the reactants are brought together, the rate at which the heat of reaction is furnished or removed and the rate at which the reaction products are removed. In conventional static fluidized beds, the rate at which the fluidizing gaseous agent, which by and large also serves as the fluidizing agent, can be blown through the bed is limited by the fact that the fluidizing currents within the fluidized zone are vertical, i.e., only the gravity force on the bed particles opposes the balancing gaseous agent force needed to maintain fluidization. If the force opposing the balancing fluidizing force could be increased, then the fluidizing agent flow rate through the bed and the reaction rate of the system would be increased. This can be accomplished using rotating fluidized beds wherein the fluidizing gaseous agent forced through the bed from its periphery opposes the centrifugal force tending to throw the bed particles outwardly from the bed axis of rotation toward the bed periphery. The extent of the centrifugal force and, thus, of the opposing fluidizing gaseous agent rate can be controlled by controlling the speed of bed rotation.
The principal advantages of a rotating fluidized bed over a static fluidized bed are the reduced volume necessary to produce a specified energy density, e.g., energy densities of 200 megawatts per cubic meter are attainable; improved chemical reaction rate, primarily attributable to the rapid diffusion of reactants, enhanced mixing and rapid removal of chemical reaction products; and very high heat transfer rates, the values for the heat transfer coefficient between the bed and surfaces within the bed having been reported on the order of 240 BTU/hr-ft.sup.2 -.degree.F. compared with 180 BTU/hr-ft.sup.2 -.degree.F. in a static fluidized bed and 10-25 BTU/hr-ft.sup.2 -.degree.F. in conventional, nonfluidized bed environments.
A form of rotating fluidized bed system has been suggested by J. Swithenbank in his article "Rotating Fluidized Bed Combustor/Gasifier". The Swithenbank system includes a vertical shaft around which rotates a generally cylindrical combustor using natural gas as the fuel. The gas is introduced at the center of the combustor, i.e., along the axis of rotation, and is mixed with fluidizing air forced through the bed particles from the bed periphery toward the center. The bed, which is heated by the combustion heat generated and the mixing action accompanying rotation, preheats the entering fluidizing air. Most of the combustion between the heated air and the natural gas appears to occur outside, rather than within, the bed itself. Cooling coils passing through the bed carry air which is heated by the combustion and serve to control the bed and exhaust gas temperature. Swithenbank states that his combustion system may be operated by burning or gasifying coal granules in the fluidized bed, but discloses no combustor configuration suitable for use with coal fuels. Moreover, Swithenbank's configuration, requiring introduction of the fuel along the axis of rotation, detracts from the attainment of maximum energy density because it diminishes the compactness of the system. See also, Demircan et al, Rotating Fluidized Bed Combustor, published in "Fluidization" by Cambridge University Press (1978). Other publications of interest in connection with the heat transfer and combustion characteristics of natural gas fueled rotating fluidized beds are J. Broughton and G. E. Elliott, Heat Transfer and Combustion in Centrifugal Fluidized Bed, I. Chem E. Symposium Series No. 43 (paper presented at June 1975 meeting) and G. C. Lindauer et al, Experimental Studies on High Gravity Rotating Fluidized Bed, U.S. Atomic Energy Commission, BNL-50013 (September 1966).
One well known process for the production of hydrogen is the steam-iron process wherein iron (Fe) is oxidized by steam (H.sub.2 O) to ferric oxide (Fe.sub.3 O.sub.4) and hydrogen is produced. This process, when practiced in the conventional manner using conventional retorts, is believed to be superior, to other hydrogen production processes such as the electrothermal and steam-oxygen process. Nevertheless, the steam-iron process as presently practiced is relatively low in thermal efficiency, purity of hydrogen-rich gases produced and reaction rates and relatively uneconomical in terms of the bulk and weight of equipment necessary.
Accordingly, the present invention is directed to overcoming one or more of the problems as set forth above.